1. Field of the Invention
The present invention relates to control on deflection amount of a transfer material on which an image is formed by an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, a transfer material is conveyed in contact with a plurality of rotatable members. In that case, it is known that a deflection amount (referred to as a loop amount) of the transfer material being conveyed is controlled to be constant so that the transfer material is neither pulled nor pushed in the conveyance direction. The sensors for detecting the loop amount include a sensor consisting of a photo-interrupter and a mechanism flag and a sensor employing a reflective optical sensor, for example. FIG. 2B is a diagram illustrating a configuration and operation of an apparatus for detecting a loop amount with a reflective optical sensor. An optical sensor can measure a distance to a transfer material by emitting light from an emission part 50a and receiving the light reflected from the top surface of the transfer material at a light reception part 50b. With the measurement results, the apparatus successively detects the loop amount of the transfer material and controls a drive unit of a secondary transfer part 20 or a fixing part 34 so that the loop amount of the transfer material becomes the same as the preset loop amount (see Japanese Patent Application Laid-Open No. 2000-89605 and Japanese Patent Application Laid-Open No. 2001-106380, for example).
A problem emerged, however, in that when a reflective optical sensor is used for detecting the loop of the transfer material, the feature of the sensor leads to an error in the loop detection result depending on the density of the top surface of the transfer material. For example, some optical sensors triangulate the distance to an object by receiving an infrared ray irradiated from the emission part at the light reception part made of a Position Sensitive Detector (PSD). In that type of optical sensor, a change in the reflection amount due to such reasons as a change in the density of the top surface of the transfer material or the image formed on the top surface of the transfer material leads to a difference between the actual distance to the transfer material and the detected distance to the transfer material. Therefore, there is a problem in that a reflective optical sensor used for detecting a loop amount cannot give a correct loop amount for some transfer materials or some images transferred on the transfer material.